ABSTRACT Despite improvements in infection control, one child dies every minute from malaria, which is caused by infection by Plasmodium parasites. Children develop immunity to malaria slowly after multiple exposures, and lose immunity in the absence of continuous exposure. A lack of critical knowledge about the antibody response that controls this infection hinders vaccine development. Early IFN-? from Th1 cells limits early parasite growth and correlates with protection. CD4 T helper cells (Th) also promote parasite-specific antibody, which we have proposed is primarily required for full parasite clearance, which takes over a month. IL-21 promotes B cell responses and is also required for clearance of Plasmodium. Splenic germinal centers optimize B:T cell interactions that promote isotype switching and affinity maturation of antibodies, and long-lived plasma cell generation. However, the appearance of GCs is delayed until late in infection by the Th1 response. This slow response may explain the delay of development of immunity observed in the field. However, there is a significant extrafollicular antibody response that is faster, and actually corresponds with dramatic control of the parasite. In addition, our preliminary data showing that knockout mice lacking germinal centers (GC) control infection, led us to develop a working model of protection consisting of three phases. First, Th1 cytokines limit parasite growth; next, GC-independent factors control the parasite to low, but chronic levels without pathology; when finally, GCs play a role in complete clearance. Yet, the types of Th cells required, and the dominant mechanisms of antibody-mediated parasite killing are not yet clear, especially in the control phase. The role of IL-21 in an extrafollicular response has not been sufficiently studied to understand the mechanisms or importance. Therefore, we hypothesize that GC-independent mechanisms driven by IL-21 make a significant contribution to the control of Plasmodium infection. To test this, in Aim 1, we will compare the contribution of GCs and IL-21 for control of parasite. We will determine the importance of GC-driven antibody changes by using novel mouse models separately deficient in isotype switching and affinity maturation. There are also important unanswered questions about the type of Th cells that help B cells make antibody in malaria infection. We and others have recently discovered that the effector T helper cell response to mouse and human malaria is composed largely of a hybrid IFN-?+ IL-21+ Th1/Tfh cell type that has all the hallmarks of both Th1 and Tfh. While IFN-? was recently shown to inhibit GC formation, the role of hybrid IFN-?+/IL-21+ T cells' contribution to antibody production in vivo, and their role in protection from parasitemia and pathology in malaria, have not been tested. Therefore, In Aim 2 we will evaluate the efficacy of hybrid IFN-?+/IL-21+ Th1/Tfh in protection, by sorting them using cytokine reporter mice, and determine the phases of infection where IL-21 is critical. Understanding the mechanisms of both the control phase and final clearance is critical because control of parasite corresponds with termination of malaria pathology and may suggest novel vaccine strategies.